This application relates generally to gas turbine engines and, more particularly, to a method and apparatus for monitoring gas turbine engine igniter performance.
At least some known gas turbine engines include a compressor, a combustor, an ignition system, and a turbine. Airflow entering the compressor is compressed and directed to the combustor where it is mixed with fuel and ignited using the ignition system, producing hot combustion gases used to drive the turbine.
At least some known ignition systems generate a relatively constant voltage that is output to an igniter. The igniter generates a spark across the igniter gap to initiate combustion of the fuel-air mixture within the combustor. Once the ignition system starts the combustion process, the continuous, controlled inflow of fuel to the combustor is generally sufficient to maintain the combustion process and the power derived from that combustion process.
More specifically, a sufficiently high voltage is required for a spark to jump the igniter gap at a predetermined ambient pressure within the combustor. As the ambient pressure within the combustor increases, the minimum voltage required to produce a spark also increases. During the spark event, a portion of the fuel/air mixture residing within the igniter gap, i.e., the spark path, is ionized such that a spark can occur. As the ambient pressure increases, the quantity of molecules of the fuel/air mixture that must be ionized also increases. Thus, to generate a spark, a voltage supplied to the igniter must also be increased to facilitate ionizing these additional gas molecules and thereby to produce the spark.
If the ambient pressure within the combustor is too high for a given voltage and/or spark gap, the igniter will not spark. More specifically, the ambient pressure within the combustor may eventually reach a pressure at which the igniter tip voltage becomes unable to ionize the gas across a given spark gap, and thus unable to produce a spark, referred to herein as a “quench pressure”. Moreover, the spark gap, between the electrode and the grounded side, widens as a portion of the igniter tip material is liberated with each spark. As the spark gap increases the useful remaining life of the igniter is reduced, and a drop in the quench pressure occurs. As a result, when the igniter is supplied with a relatively fixed voltage, any increase in the igniter spark gap results in a corresponding decrease in the quench pressure.
At least some known igniters have a life expectancy that is inversely proportional to the time the igniter is energized. For example, each time the igniter is energized, the remaining life of the igniter is reduced. Other factors that may facilitate reducing the life expectancy of the igniter include factors such as, an operating voltage supplied to the igniter, an igniter material, and/or a corrosiveness and temperature of the operational environment. Therefore, several independent and varying factors can effect the life expectancy of each igniter during its period of use.
At least some known gas turbine engines include igniters that have operated in excess of approximately 12,000 hours. However, estimating the life expectancy of a new igniter and/or the remaining life of a used igniter is problematic. As such, at least some known igniters are replaced when the igniter has been operated for a predetermined quantity of engine hours. For example, at least some known igniters are replaced during a routine scheduled maintenance event. Therefore, at least some igniters are replaced prior to the igniter reaching the end of its service life, whereas other igniters may be replaced after the igniter has reached the end of its service life. Replacing an igniter prior to the end of its service life increases the maintenance and operating costs of the gas turbine engine. Whereas, replacing the igniter at the end of its service life increases the possibility that the engine may not start during normal operation.